In recent years, in association with progress in the field of image processing technology, now it is possible to make so elaborate copies of bills that copied bills made by a color copying machine can not be distinguished from actual ones. For this reason, now serious efforts are made to develop a specific document discriminating apparatus which can discriminate specific document of bills, securities and the equivalent for preventing the specific document being used for practical purposes.
In a specific document discriminating apparatus, the technology based on the pattern-matching method as described, for instance, in Japanese Patent Laid Open Publication No. 83571/1990 with the title of "Image Recognizing Apparatus" has generally been used. In this apparatus, determination as to whether a document is a specific one or not is made by comparing input image data to pattern data registered previously.
With the conventional type of technology as described above, a particular portion of a specific document is read by checking with visible rays, and read image data is collated and discriminated by means of the pattern matching method, but in this case, however, successful recognition ratio is not 100%, and also general document often includes areas similar to the specific portion of specific document, so it is not always possible to distinguish a specific document from a general document.
If for instance, a precision in discrimination (a threshold value used for discrimination) is raised to prevent a specific document from being overlooked, a percentage of error recognition in which a general document is read as a specific document increases, and on the contrary if a precision in discrimination is lowered to prevent a general document from being read as a specific document, a percentage of recognition in which a specific document is read as a general document goes higher. It is an object of the present invention to make it possible to accurately distinguish a specific document from a general document without fail.